1. Field of the Invention
The present invention relates to an apparatus for electrically processing, for color correction, a video signal used for a video printer.
2. Description of the Background Art
A video printer prints out color images, e.g. images taken by video cameras or images form public broadcast television. Not all of the images, however, have necessarily been obtained under ideal photographic conditions. Some images are dark due to shortage of light, for example. Others have poorly-balanced colors, or insufficient chrominance. It is therefore necessary to improve quality of the color images through an appropriate color image treatment.
The color image treatment in video printers includes 2 stages: image adjustment applied to a video signal and color correction for respective color separated images.
Firstly, a user of the video printer can adjust the hue and/or chrominance of the color images by manipulating a `TINT` and/or a `SATURATION` knob, just like in a color television receiver. The hue of the color image is altered by shifting the relative phase between the color sub-carrier and color carrier signals at the time of color demodulation. The chrominance is altered by changing the amplitude ratio between the color sub-carrier and color carrier signals. With this adjusting method, the hue and/or chrominance of every color component in the image are changed together. A specific color cannot be selectively adjusted regarding its hue and/or chrominance.
After the hue and/or chrominance have been adjusted as described above, the video signal is subjected to color separation by a luminance/chrominance separation circuit and a decoder circuit, and RGB (Red, Green, Blue) signals are obtained. Respective color-separated signals are then subjected to masking treatment by a matrix conversion circuit for color correction.
Inks used for the video printer include quantities of unnecessary light-absorbing dyes and therefore develop a tint or hue different from the ideal one. If the color separated signals are supplied to the printer without color correction, a vivid color image comparable to the picture on a color TV receiver cannot be reproduced due to the unnecessary dyes included in the ink. The above-mentioned color correction for the color-separated signals is therefore necessary. However, the user of the video printer cannot alter the constants in the matrix conversion circuit as he/she desires, because these constants are permanent parameters of the video printer determined in its design stage.
On the other hand, colors of skin S and green f are important colors for which man has an intrinsic feeling and personal preferences. It is therefore desirable for the video printer to be capable of adjusting the hue, chrominance, and luminance in the vicinity of the colors S and f, independently. Conventional video printers, however, have no such a capability, for the reasons explained above.